Why a mirror?

OK, I must be missing something here. While Sony's semi-transparent mirror is interesting (though not new - Canon did it at some 30 or 40 years ago with the Pelix, and has done it in a number of different models), I don't understand why the A77 has a mirror at all.

SLRs have a mirror because you need to be able to get light to the optical viewfinder AND to the film / sensor. But the A77 has an EVF, so I assume it's getting the viewfinder image off the sensor. So why the mirror? What am I missing?